This invention relates to apparatus for converting signal information that is made available by a pulse-echo surveillance system or other source at a particular rate and in a particular coordinate format into signals compatible with various visual imaging or display devices that operate at a different rate and with reference to a different coordinate format. More specifically, this invention relates to digital scan converters of the type that convert analog input data which is referenced to a polar coordinate system into a Cartesian coordinate format to thereby permit visual information display with conventional apparatus such as a television receiver.
There are many situations in which information is gathered or received at a particular rate and with reference to one type of coordinate system wherein it is desired or necessary to convert the received information into a different coordinate format for utilization at a rate that may be substantially different than the rate at which the information is gathered or received. For example, various surveillance apparatus such as slow scan radars, ultrasonic imaging and sonar systems scan a sectorial region of an object or the surrounding environment by emitting a series of energy pulses which propagate outwardly along angularly spaced apart, radially directed paths and by detecting return or echo signals that occur when the emitted pulses are scattered by reflective structure that lies within the propagation paths. Since the time which elapses between the emission of a particular energy pulse and the associated return signal (or any portion thereof) is related to the distance between the structure causing the signal reflection and the source of the energy, the return signal can be processed to provide a two-dimensional visual representation of the scanned region. In this respect and regardless of the particular structure employed, the operation of such surveillance systems can be conceptually considered as the rotation of a single transducer element through a total scanning angle by repeatedly moving the transducer through small incremental angular steps while emitting a pulse of energy and receiving the associated return signal at each angular position of the transducer. Thus, the signals provided by sector scanning surveillance systems are essentially based on a polar coordinate system wherein the emitted energy can be mathematically modeled as a point source that is located at the origin of the polar coordinate system; the value of the angular or azimuthal coordinate, .theta., expresses the direction of an emitted pulse of energy and its associated return signal; and the value of the radial coordinate, r, expresses the radial distance between the transducer and the structure causing the reflective energy scattering. On the other hand, the operation of most modern display devices can be considered to be based on a Cartesian coordinate system in which the image being displayed is generated by rapidly and successively producing small incremental regions of illumination on the face of a cathode ray tube or a mosaic of electroluminescent elements while modulating the intensity of the illumination in accordance with the image being formed. In this respect, in order to provide high image resolution the incremental elements of such display devices must be relatively small and must be successively energized at a relatively high rate in order to update or refresh the visual display at a rate which provides "flicker free" viewing. The rate at which return signals are provided by a surveillance system may, of course, differ substantially from the rates at which flicker free visual displays can be produced and, in many situations, the display rate may be fixed by convention or other design constraints. For example, in a conventional black and white television display an electron beam impinges on a phosphorous coating that is applied to the rear surface of the television viewing screen to produce a small spot of illumination having an intensity that is proportional to the electron beam current. To provide the displayed image, the electron beam is swept horizontally across the face of the screen at a rate of 15,750 sweeps per second while being swept from the top to the bottom of the screen at a 60 Hertz rate. Therefore, each image frame is comprised of 525 horizontal sweeps during each of which the electron beam sweeps across the screen to excite the phosphorous coating during approximately a 55 microsecond interval and retraces to the original vertical border of the screen in approximately 8 microseconds. With reference to vertical displacement, one-half of the horizontal sweeps are effected during one period of the 60 Hertz vertical scanning rate and the final one-half of the horizontal sweeps are effected in spatial alternation with the previously generated horizontal sweeps during the next period of the 60 Hertz vertical sweep rate to thereby completely update or refresh the image being displayed at a rate of approximately 30 Hertz.
Various digital scan converters have been proposed for accepting data at the rate at which it is made available by a surveillance system or other source that, in effect, operates with reference to a polar coordinate system and providing the data at a different rate while simultaneously facilitating display of the data by a device that, in effect, operates with respect to a Cartesian coordinate system. Basically, such prior art digital scan converters sample each reflected signal at a predetermined rate to provide a set of digitally encoded signals (digital words) that represent the amount of reflection occurring from spaced apart locations along the radially extending scan line that was traversed by both the transmitted energy pulse and the associated return signal. Since, as was previously noted, sector scanning surveillance systems sequentially emit pulses along a plurality of angularly oriented scan lines in order to provide a two-dimensional surveillance region of sectorial geometry, one complete scanning sequence produces a plurality of sets of digital words wherein each set of digital words represents reflections occurring along a particular scan line and successive sets of digital words occur at the pulse repetition rate of the surveillance system. The sets of digital words are then stored in a memory device such as a random access memory (RAM) in a format which identifies each digital word with a corresponding spatial position within the scan region. The stored digital words are then read from the memory device at a rate compatible with the display apparatus of interest and are typically converted to an analog signal suitable for use with such display apparatus.
One example of a prior art digital scan converter of the abovedescribed type is disclosed in Heard et al., U.S. Pat. No. 3,810,174, in which an analog-to-digital (A/D) converter is clocked at a constant rate representing a desired range increment to produce digitized signals which are accumulated in a buffer memory until a data group corresponding to one complete scan line of an associated radar system is available and a preassigned area within a first track of a multitrack drum-type magnetic memory is available for storing that data group. When both conditions are satisfied, the accumulated data is written into the preassigned region of the drum memory, which also includes a second track that carries the address of that scan line, i.e., the azimuthal angle of a polar coordinate system when the data being stored results from a sector scanning surveillance system of the type hereinbefore described. Recorded data is then cyclically read from the drum memory and converted to an analog signal at a rate suitable for operating the x and y deflection and the z axis intensity circuits of a conventional cathode ray tube display apparatus. In this respect and in order to display the information being read from the drum memory with a proper Cartesian orientation, the digitally encoded addresses of the data being displayed are converted to an analog ramp signal so that the instantaneous amplitude of the ramp is representative of the angular azimuthal coordinate of the particular group of data being displayed. Two analog function generators respectively supply a signal representative of the sine and cosine of that particular azimuthal angle. These signals are respectively coupled to the x and y deflection circuits of the cathode ray tube. According to the above-referenced Heard et al. patent, this modulation of the cathode ray tube x and y deflection signals causes the electron beam to be continuously displaced so that the resulting visual display exhibits proper polar coordinate perspective.
With respect to the display of sector shaped data fields, it can be noted that a digital scan converter of the type disclosed in the above-referenced patent to Heard et al. is not suitable for use with the display format utilized in conventional television systems since, as previously described, the electron beam of a television display device cannot be deflected in the vertical direction during each of the horizontal sweep operations.
U.S. Pat. No. 3,765,018, also issued to Heard et al., and U.S. Pat. No. 4,065,770, issued to Berry, describe prior art digital scan converters that are similar to the apparatus disclosed in the previously referenced patent to Heard et al. in that: (1) each return signal of a sector scanning radar system is effectively sampled and digitized by an A/D converter which operates at a constant rate determined by the scan rate of the associated radar system; (2) the digital words resulting from the A/D conversion, which represent reflections originating at equally spaced apart locations along a series of angularly oriented scan lines are stored in a memory; and (3) the stored data is accessed at a rate compatible with a display device such as a conventional television receiver and is converted to an analog signal for driving such a display device. In the digital scan converters of both of these references, the memory is a random access memory (RAM), and conversion between a polar coordinate system that is associated with the radar system and a Cartesian coordinate system in which lines having a constant y coordinate horizontally traverse the image display is effected as the digital words are written into the storage locations of the RAM. In particular, as the A/D converter supplies each digital word, the digital scan converters of the these references determine the azimuthal coordinate .theta. and the radial coordinate r which correspond to the associated unique physical location along that particular scan line. The projection of the radial coordinate on both the x and y axis of the desired Cartesian system is then calculated and the digital word provided by the A/D converter is stored in the RAM at a storage location that can be addressed in terms of the resulting x and y coordinate values. In the apparatus of the Heard et al. patent, the RAM is, in essence, arranged as a rectangular array that corresponds to the Cartesian display format and the horizontal traces of the displayed image are produced by successively accessing adjacent rows of the memory array. In the patent to Berry, the sector scanning surveillance system is a ground mapping radar that is mounted in a moving aircraft or other vehicle so that the polar coordinate system associated with the input data moves relative to a fixed point on the ground. To permit convenient display relative to the Cartesian coordinate system of a television screen located within the aircraft, the above-mentioned polar-to-Cartesian coordinate transformation is effected relative to a Cartesian reference that remains fixed relative to the ground and a fairly complex addressing scheme is implemented which compensates for movement of the aircraft relative to the ground.
Nevin, U.S. Pat. No. 4,002,827, discloses yet another digital scan converter that includes a A/D converter, a RAM, and a D/A converter wherein the reference signal associated with each radially extending scan line of a sector scan radar system is digitized, stored in the RAM at a rate determined by the radar scanning rate and the stored data is accessed at a rate compatible with a display device that operates in a Cartesian coordinate format. In the apparatus disclosed by Nevin, the conversion from polar coordinate format to Cartesian coordinate format is effected as the digital words supplied by the A/D converter are loaded into the RAM so that the RAM can be considered to be a rectangular matrix having storage locations corresponding to incremental cells of the surveillance region to be displayed. Although this technique is also employed in previously referenced U.S. Pat. Nos. 3,765,018 and 4,065,770, the apparatus disclosed by Nevin does not operate at a constant sampling rate, but is driven by a clock circuit which exhibits a pulse repetition rate that is proportional to the cosine of the azimuthal angle defining the polar orientation of the scan line associated with the return signal being processed. This means that the return signal associated with a particular scan line is digitized at a slower rate than adjacent scan lines having a smaller angular coordinate and, relative to the Cartesian coordinate system of the display device, the successive digital words of each reflected signal will lie on successive, parallel traces that are equally spaced apart and exhibit a constant y coordinate. To place each digital word into a storage location of the RAM having the proper orientation relative to the x axis of the desired Cartesian coordinate system, the apparatus disclosed by Nevin operates in the manner discussed relative to the previously referenced U.S. Pat. Nos. 3,765,018 and 4,065,770. That is, as the A/D converter supplies a digital word, the r coordinate of the corresponding location along the scan line being processed is determined and multiplied by the sine of the azimuthal angle to derive an address signal that will cause the digital word to be placed in the proper storage location of the RAM. Since, in the apparatus disclosed by Nevin, each row of the rectangular array of the RAM corresponds to one complete horizontal trace of the desired display, an analog signal compatible with conventional TV display devices is formed by successively reading the stored data on a row-by-row basis with each row of data being clocked into a D/A converter at a constant rate that is compatible with the conventional TV horizontal sweep rate. During this process, a conventional composite television sync signal is added to the analog video information.
Although the apparatus disclosed in the above-referenced U.S. Pat. Nos. 3,765,018; 4,002,827 and 4,065,770 may provide satisfactory results under some circumstances, several disadvantages and drawbacks are encountered. First, since the digital scan converters disclosed in these references perform the desired polar-to-Cartesian format conversion as surveillance information is entered into a memory which includes an adequate number of storage locations to represent each small incremental region of spatial cell of the face of the display device, the memory devices utilized therein must have substantial storage capability. For example, in the embodiment of a scan converter disclosed by Nevin for displaying a sectorial surveillance region on a television screen, the display region of the television screen is considered to comprise 512 horizontal traces consisting of 512 equally spaced apart "dots" so that the face of the television screen is, in effect, an array containing 262,144 such dots. Thus, if the surveillance information is encoded into four-bit digital words by the A/D converter, a RAM having a 1 megabit storage capability is required even though relatively few of the storage locations will actually hold signal information. In particular, in displaying a sectorial surveillance region on the substantially rectangular face of a television screen, a major portion of the screen area will be outside of the imaged surveillance sector and hence the RAM storage locations corresponding to this region of the television screen are not utilized to store image information. However, as previously noted, these storage locations are required in the apparatus disclosed by Nevin so that when data is read from memory at a constant clock rate the D/A converter will supply a television compatible analog signal having proper amplitude versus time characteristics.
The apparatus disclosed in the above-referenced patents not only utilize more memory than is necessary to store the desired surveillance information, but implementation of the disclosed apparatus requires a substantial amount of rather complex circuit arrangements. In this regard, the apparatus disclosed in both the Heard et al. patent and the Berry patent require calculations based on the sine and cosine of a scan line in order to generate the memory address for each digital word being supplied by the A/D converters and since the resulting memory addresses do not correspond to a predetermined mathematical series, conventional counting techniques commonly utilized in memory addressing operations cannot be employed. Although the special calculation of a y coordinate or row memory address is not required in the apparatus disclosed by Nevin, it is still necessary to utilize the sine of the angular coordinate of each scan line and generate an appropriate x or column address for each digital word supplied by the A/D converter in order to complete the polar-to-Cartesian format conversion as data is entered into the RAM.
The addressing requirements of the above-referenced patents not only mean that the digital scan converters of the references must include an arithmetic processor or other rather complex computational logic arrangement, but each computation requires a discrete amount of time. In particular, it is usually desired or necessary to display the imaged portion of the surveillance region in real time (i.e., as the surveillance system provides current input data) and with the loss of little or no signal information. In this regard, since a television system operates at a relatively fast scanning rate, little time is available for processing the signals supplied by the surveillance system and, although buffering of the data supplied to the RAM and data read from the RAM is useful in processing the necessary digital information in the time available, the need to perform polar to rectangular address calculations can impose severe design constraints. Thus, when a large amount of digital information is to be processed, one implementing the digital scan converters of the references would seem to be faced with a choice of either increasing the capability and complexity of the memory buffering stages or sacrificing the quality of the displayed image. That is, under many circumstances it would appear that a tradeoff must be made in implementing the apparatus of the references as between increasing the structural complexity of the digital scan converter and possibly sacrificing the loss of some image information.
Accordingly, it is an object of this invention to provide an improved digital scan converter for use with apparatus such as sector scanning surveillance systems wherein image signal information is provided that is compatible with a display device operating in a Cartesian coordinate format.
It is another object of this invention to provide a digital scan converter which processes analog signals representing information relative to a polar coordinate format and supplied at a particular rate to form signals having the same information content for use with a display device that is based on a Cartesian coordinate format and operates at a rate that is independent of the rate at which analog signals are supplied to the digital scan converter.
It is a further object of this invention to provide a digital scan converter for use in displaying the sectorial surveillance region of a sector scanning surveillance system with conventional television apparatus wherein the digital scan converter operates on a real time basis to supply an image of the region presently being scanned by the surveillance system.
Still further, it is an object of this invention to provide a digital scan converter wherein the scan converter memory requirements are minimized relative to the amount of surveillance information being handled.
Further yet it is an object of this invention to provide a digital scan converter, which not only meets the above-stated objects, but is of minimal structural complexity and hence relatively economical to fabricate.